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Re: Basic ignition question (long)
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Posted by Eric Law [Email] (more from Eric Law) on Tue, 20 May 2003 12:42:02 Share Post by Email
In Reply to: Basic ignition question. The only stupid question is, Trobinson9kT, Sun, 18 May 2003 20:11:57
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Tom,

Well I was hoping someone with more of a clue would answer this, but no such luck so I'll take a crack at it... ;^)

I think there are 2 separate mechanisms at work.

First, advancing the timing increases the peak cylinder pressure during the combustion cycle. This is because the fuel is burning while the piston is higher up, thus there's less room for it to expand. And as the flame front expands away from the piston, the increasing pressure can cause the mixture to spontaneously combust in other areas of the chamber, which constitutes detonation.

The second effect is that it causes everything to happen earlier in the engine's cycle. Combustion starts earlier, and thus ends earlier. Because it ends earlier relative to the opening of the exhaust valve, the combustion gases have more time to exit the cylinder. Thus, when the intake opens, there are less of these gases remaining in the cylinder. The residual gases have (in a small way) the same effect as EGR - they prevent the engine from detonating. So less residual gases = increased tendency to detonate.

Confused yet? I know I am! :^)

Eric Law

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